1. Field of the Invention
The present invention relates to an exhaust heat exchanger.
2. Description of Related Art
JP-A-2003-106785 discloses an exhaust heat exchanger having a water tank, tubes arranged in the water tank, an inlet gas tank, an outlet gas tank, and a flange. The tube defines a passage of exhaust gas recirculation (EGR) gas. EGR gas is recirculated into an engine after exhausted from the engine. The tube has an inner fin inside of the passage. The inlet gas tank is connected to an end of the water tank, and distributes EGR gas into the tubes. The outlet gas tank is connected to the other end of the water tank, and gathers EGR gas flowing out of the tubes. The flange is fixed to each of the gas tanks, and an EGR gas pipe is connected to the flange. An inlet water pipe is connected to the water tank, and cooling water flows into the water tank through the inlet water pipe. An outlet water pipe is connected to the water tank, and cooling water flows out, of the water tank through the outlet water pipe. A core plate is arranged at longitudinal ends of the tubes, so that inside of the water tank and inside of the gas tank are separated from each other by the core plate. EGR gas passing through the tube is cooled by cooling water flowing outside of the tubes in the water tank.
The tubes, the fins, the gas tanks, the flange, the water pipes and the core plate are all made of stainless steel, and are integrally brazed with each other.
Aluminum is widely used as a material for producing a heat exchanger, and is light and cheap compared with stainless steel. Further, brazing property of aluminum is better than that of stainless steel. However, heat resisting property of aluminum is lower than that of stainless steel.
A temperature of EGR gas is high at an inlet side of the heat exchanger. Therefore, mechanical connection strength between the flange and the EGR gas pipe is easily lowered, and strength of the inlet gas tank may not be secured. Further, difference of thermal expansion is generated between the inlet gas tank and the water tank, because a temperature difference is generated between the inlet gas tank that is exposed to hot EGR gas and the water tank that is exposed to cold cooling water. Thermal expansion coefficient of aluminum is larger than that of stainless steel. Therefore, thermal stress generated at a connection between the inlet gas tank and the water tank is large when the heat exchanger is made of aluminum, compared with a case where the heat exchanger is made of stainless steel. That is, if the heat exchanger is produced with a material having a low heat resisting property, temperature lowering is necessary for the inlet gas tank and the flange.